TRAIL was identified in the 90s of the last century. Soon after TRAIL was discovered, the attention was paid to its potential as an anti-cancer agent for cancer therapy. This was based on the ability of TRAIL to selectively kill tumor cells but not normal cells. Importantly, anti-tumor efficacy of TRAIL can be significantly enhanced by many current cancer therapies (for example, chemotherapy and radiation therapy). On the other hand, TRAIL can sensitize tumor cells and increase the susceptibility of tumor cells to chemotherapy and radiation therapy. Therefore, the combination of TRAIL with chemotherapy and radiation therapy has been thought to be a very effective anti-tumor therapy in the future.
TRAIL is a member of the TNF family of proteins. A feature of some proteins of this family is their ability to induce apoptosis such as TNF-α and Fas ligand. However, due to their toxic side effect, TNF-α and Fas ligand have no value for clinical application. In contrast, TRAIL exhibits a selective killing to tumor cells, its clinical value is obvious. To date, five receptors for TRAIL have been identified, two of which, DR4 (TRAIL-R1) and DR5 (TRAIL-R2), are capable of transducing the apoptosis signal while the other three DcR1 (TRAIL-R3), DcR2 (TRAIL-R4), and osteoprotegerin (OPG) do not transduce the apoptosis signal. All five receptors for TRAIL share significant homology in their extracellular ligand binding domains. The intracellular segments of both DR4 and DR5 contain a conserved functional domain, so called “death domain”, which is responsible for transducing apoptosis signals.
After several years of study, the major biological function of TRAIL has been well known. TRAIL plays an important role in immune surveillance to tumor cells. Activated T lymphocytes and NK cells express high levels of TRAIL, which arms these immune competent cells to kill tumor cells. Animal studies indicate that knockout of TRAIL leads to increased incidence of tumor with age. Therefore, defective or insufficient expression of TRAIL might be a critical factor of tumorigenesis.
Because the apoptosis-inducing function of TRAIL is mediated by its receptors, research on TRAIL receptor system has been extensive. Early studies suggest that many normal cells may express the death receptors (TRAIL-R1 and TRAIL-R2) for TRAIL at the transcriptional level. With the availability of anti-death receptor antibodies, it has been believed that normal cells and tissues express very low levels of cell surface TRAIL-R1 and TRAIL-R2. In contrast, normal cells and tissues may express high levels of TRAIL-R3 and TRAIL-R4. This differential expression of different TRAIL receptors in normal cell may be a critical protective mechanism for normal cells to escape from TRAIL killing. Different from normal cells, most transformed tumor cells express high levels of TRAIL-R1 and TRAIL-R2 whereas the expression levels of TRAIL-R3 and TRAIL-R4 are very low. Thus, most tumor cells are Susceptible to TRAIL-mediated killing. The differentially expressed TRAIL receptors between normal and tumor cells well explain the selectivity of TRAIL.
Many pre-clinical studies have confirmed that TRAIL is a safe and effective therapeutic agent for treatment of cancer. It has been shown that the systemic administration of the trimerized soluble TRAIL did not cause toxicity in experimental animals yet was able to induce regression of implanted tumors. It is even more encouraging that when TRAIL is combined with chemotherapy or radiation therapy, its anti-tumor efficacy is significantly enhanced. This synergistic effect has been demonstrated by many in vitro and in vivo experiments. In addition, TRAIL can increase the sensitivity of tumor cells to chemotherapy and radiation therapy. Because tumor cell resistance to chemotherapy and radiation therapy has been a major obstacle in treatment of cancer, the ability of TRAIL to prevent or reverse chemo or radiation resistance might be a significant advance in future cancer therapy.
However, as a therapeutic agent, TRAIL has several disadvantages. First, TRAIL has at least five receptors including both death receptors and decoy receptors, therefore lacking the selectivity to the receptors. Particularly it is hard to predict the apoptosis-inducing capability of TRAIL, when cancer cells express differentiated death receptors and decoy receptors. Second, the recombinant TRAIL has very short in vivo half-life, which limits the effective dose and anti-cancer efficacy of TRAIL in vivo. It is not convenient that patients usually receive repeated and large doses of TRAIL. Third, it is concerned that certain forms of recombinant TRAIL have potential hepatocyte toxicity.
These limitations of TRAIL as a therapeutic agent led to development of the alternatives to TRAIL. Monoclonal antibodies may selectively target the death receptors of TRAIL, which might be a more effective and safe strategy to cancer treatment.
During 25 years since the first monoclonal antibody was generated, monoclonal antibodies have demonstrated a great impact in cancer treatment. Most of those clinically effective monoclonal antibodies target antigens or receptors that are highly expressed on cancer cell surface, and block the growth signals required for tumor growth. These antibodies kill tumor cells through activation of compliments and antibody-dependent cytotoxicity (ADCC). In addition, monoclonal antibodies may be used as a tracing molecule, when conjugated with radioisotopes, toxins and drugs, to bring these therapeutic agents to cancer tissues and enhance anti-cancer efficacy.
The generation of TRAIL-R1 or TRAIL-R2 specific monoclonal antibody to replace TRAIL for cancer therapy has been successful. Several such antibodies have been in clinical trials. Preliminary results demonstrate that these antibodies not only have strong anticancer efficacy but also are safe compared to TRAIL.
Japanese pharmaceutical company, Sankyo, first developed an anti-TRAIL-R2 antibody, TRA-8. Ichikawa et al. used TRAIL-R2-Fc fusion protein as immunogen to immunize Balb/c mice. While TRA-8 did not induce apoptosis of normal cells, many tumor cells were highly susceptible to TRA-8-induced apoptosis. Although mRNA of TRAIL-R2 is widely distributed in normal tissues, the TRAIL-R2 protein was not detectable in normal tissues including live, lung, breast, kidney, spleen, ovary, hear and pancreas. However, cancer cells in these tissues expressed high levels of TRAIL-R2 protein. In addition, normal glial cells and peripheral blood cells expressed very low levels of TRAIL-R2, and are not susceptible to TRA-8-induced apoptosis, whereas gliloma cells and leukemia cells expressed high levels and are very susceptible to TRA-8-induced apoptosis. TRA-8 also exhibited several folds higher apoptosis-inducing capability than TRAIL in induction of apoptosis of tumor cells. Importantly, TRA-8 did not induce apoptosis of normal hepatocytes. When combined with chemotherapy or radiation therapy, the anti-cancer efficacy of TRA-8 is significantly enhanced. TRA-8 is currently in phase I clinical trial.
Human Genome Sciences carried out phase I trial of an anti-TRAIL-R1 antibody. Preliminary data indicate that patients well tolerated and the positive response was observed in several patients, suggesting that anti-TRAIL-R1 is a safe and effective therapeutic agent.
Many antibodies that are capable of inducing apoptosis of tumor cells are specific either for TRAIL-R1 or TRAIL-R2. A bispecific antibody to TRAIL-R1 and TRAIL-R2 has also been reported. (Lynch, US 2002/0155109). Because tumor cells may selectively express only one type of death receptors, therefore, these antibodies have a limited spectrum and unable to target all tumor cells. Meanwhile, because cancer cells may differentially express two types of the receptors and have a prefer signal transduction, the killing activity of these antibodies varies greatly. Accordingly, there is a need in the art for additional anti-TRAIL receptor antibodies to be used for TRAIL receptor detection and modulation of TRAIL receptor-mediated function.